(a) Field of the invention:
The present invention relates to an endoscope and more specifically to an optical system for endoscopes.
(b) Description of the prior art:
Most of the endoscopes currently provided as medical instruments for practical use are so designed as not to change focus point, stop aperture, focal length, etc. of the optical systems for eliminating complicated procedures during medical operations. In order to observe objects located at relatively long distances from the objective lenses (hereinafter referred to as far positions) as well as objects located as relatively short distances from the objective lenses (hereinafter referred to as near positions) in practice, the aperture stops are preliminarily stopped down to obtain required depth of field. However, brightness and depth of field are in a relationship contradictory to each other: brightness is reduced at larger depth of field, or depth of field is insufficient at higher brightness. Further, in order to realize the close-up observation hithereto and currently demanded or compensate smaller quantity of light in the thinner light guides adopted for the thinner endoscopes, persons in the art widely desire new technological development.
The automatic iris control device is available as a means to satisfy the demand and desire. Since the endoscope forms an object image by using a built-in illuminating means, the image is brighter as the distance from the objective lens to an object (hereinafter referred to as "observation distance") is shorter. The automatic iris control device is used to obtain sufficient quantity of light not by adjusting the quantity of light from the light source, but by adjusting the stop aperture of the objective lens in conjunction with the quantity of incident light. For observation of the nearer position, the automatic iris control device provides a larger depth of field by stopping down the iris and has an advantage by obtaining an image sharper than that obtained without said device. Since it is practically difficult to incorporate the distal end of an endoscope with a mechanism capable of continuously changing the stop aperture, the aperture is changed discontinuously at two or three steps.
A composition of the conventional automatic iris control device is exemplified in FIGS. 1A and 1B. The automatic iris control device consists, as shown in FIG. 1A, of an imaging lens 2 and a solid-state image sensor on the rear side (image side) of a variable stop 1. Shown in FIG. 1B is the variable stop 1 independently. Area 1a is always black, whereas area 1c is always transparent. Area 1b is transparent or black and performs the function of a variable stop.
FIG. 2A illustrates a fully opened state (I) and FIG. 2B illustrates a stopped-down state (II) respectively of the variable stop. FIG. 3 visualizes the relationship between the F number and the depth of field at operation time of the automatic iris control device.
An endoscope equipped with no automatic iris control device provides only the depth of field in the state (I). When an endoscope equipped with the automatic iris control device is set for observing the nearer position, the object is illuminated brighter and the iris is stopped down into the state (II) to properly adjust the quantity of light, thereby increasing the depth of field and widening the observation field.
However, the automatic iris control device stops down the iris only when observing the near position and provides no advantage for elongating the depth of field when observing the farther position. Since elongation .DELTA. of the depth of field on the near position side is quite insufficient, it can hardly be said that the automatic iris control device remarkably widens the practical observation range. Therefore, it was attempted to remarkably widen the practical observation range by adjusting the focus point of the objective lens onto the nearer position simultaneously with a stop-down of the iris so as to make useless the depth of field on the far position side.
For adjustment of the focus point, several methods have conventionally been contrived. For example, the position of the lens 2 is changed as shown in FIG. 4 by displacing said lens using an electromagnetic force or piezoelectric element. Further, methods have also been contrived to displace the solid-state image sensor or end surface of the image guide for changing the focus point.
However, it is difficult to build a mechanism for such displacement into a thin fiber bundle of an endoscope and focus point adjusting systems comprising a driving circuit are generally unreliable.